This invention relates to the reclaiming or reconditioning of green foundry sand used in metal casting operations.
Green sand is a mixture of sand, clay, organic adhesion promoters and water used in the formation of molds into which molten metals are poured and allowed to cool sufficiently to permit a molded metal object to be removed therefrom without injury. Typically, green sand has a moisture content of between about three and four percent and a clay content of between about five and twelve percent by weight, with the moisture and clay being essentially uniformly distributed throughout the body of the mold prior to the introduction of the metal.
After the introduction of molten metal into a mold formed of green sand, the metal slowly solidifies. After solidification, the casting is separated from the molding sand and the sand is collected for further use. However, it is not possible to reuse all the sand. Therefore, it is common practice to add at the completion of each molding cycle, a predetermined percentage of new sand and clay and to remove an identical amount of old sand from the system. An obvious disadvantage of this procedure is that part of the discarded sand includes active binder or binder subject to reactivation, and such discarded sand must be disposed of. Federal and state environmental regulations relative to the disposal of foundry sands are becoming extremely strict. This in turn has forced the closure of many foundry landfill sites and dramatically increased disposal expense. Consequently, foundries are finding it necessary to reclaim and reuse green sand as a desirable alternative to the high cost and future liability associated with disposal of waste sand even in an approved landfill.
In most green sand foundry applications, the intended use for thermally reclaimed sand is in the production of nobake cores and to satisfy a portion of the required new sand that must be added to the molding green sand. However, at the present time there is no green sand reclamation process that can consistently produce a quality reusable sand.
If green sand is reclaimed thermally, it undergoes a dramatic pH elevation during the calcination step. Calcination is heating the sand to a high temperature. It is believed that retained clay is the source of the pH elevation, because if the same type of sand is coated with an organic nobake binder, it demonstrates little or no pH change during thermal reclamation. Consequently, thermal reclamation equipment manufacturers have concentrated their efforts on reducing the clay in the reclaimed sand.
Some marginal success has been experienced in thermally reclaiming specific sands, but even extensive mechanical scrubbing has not consistently produced a quality usable, thermally reclaimed sand. Even when the amount of retained clay is reduced below 0.25 percent, the pH of the thermally reclaimed green sand is too high to allow successful rebonding of the sand in an acid set nobake binder system.
Accordingly, it is a principal object of the present invention to provide a plant and a method for reclaiming or reconditioning green foundry sand that can be used to produce nobake cores and satisfy a portion of the required new sand that must be added to the molding green sand.
It is a further object of the present invention to provide a plant and a method that will produce thermally reclaimed sand which has a pH equal or less than the pH of new sand.
It is a still further object of the present invention to provide a thermally reclaimed or reconditioned green sand which when processed in test applications, rebonds at tensile strengths about equal to that of new sand.
Extensive mechanical scrubbing of thermally reclaimed green sand, as above noted, dramatically reduces the amount of retained clay, but often results in only a minimal reduction in pH. This phenomenon motivated me to evaluate what was happening to the sand and the clay during both the thermal calcining and the mechanical scrubbing functions. By using a 1000.times. microscope, I discovered that after thermal reclamation the surfaces of the individual clay grains appeared to be very rough. This surface roughness did not appear to have been caused by abrasion.
My examination revealed that the clay grain surfaces appeared to be flaking. From this I concluded that the individual clay grain itself was undergoing an ion structure change. I also discovered that small clay particles were adhering to the sand grains. The adhesion was not mechanical; it was magnetic. Larger clay particles were free of the sand, however as respects clay particles attached to the individual sand grains, if I attempted to free such a clay particle from a sand grain, it would reattach itself to the sand grain as soon as it could make contact. I thus concluded that the ion structure change occurred only on the surfaces of the clay particles.
My observations convinced me that the typical AFS clay analysis after calcination and extensive mechanical scrubbing, is not truly representative of the amount of retained clay. I believe that the amount of retained clay is two to three times as much as an analysis shows. I believe that all the retained clay adhered to the sand grains undergoes ion change and that this is the main source of pH elevation. I, believe this is the reason why the pH of the sand does not drop proportionately to the reduction in clay.
I have discovered that it is possible to reduce the pH and free adhered clay particles from the sand by mixing an acid/water solution with the sand after calcination and mechanical scrubbing. The solution reacts with the clay adhered to the individual sand grains so as to permit the adhered clay to break free therefrom. When the sand is then dried, the clay can be released from the sand grains and the freed clay particles extracted by fluidized air.
I have found that the acid/water solution has to be adjusted in volume and pH depending on the volume of retained clay and the pH of the thermally reclaimed sand after scrubbing and separation. My invention results in a thermally reclaimed green foundry sand that has a pH equal or less than the pH of new sand and has nearly all of the clay extracted. The clay that remains in the sand has the same pH as the sand.